An X-ray computed tomography (CT) system is a device that scans a subject with X-rays and processes collected data by a computer to create an image of the inside of the subject.
Specifically, the X-ray CT system irradiates the subject with X-rays a plurality of times from different directions through an X-ray generator, and detects X-rays having passed through the subject by a plurality of X-ray detecting elements. A data acquisition unit collects detection data. After the A/D conversion of the detection data thus collected, the data acquisition unit sends the data to a console device. The console device performs preprocessing and the like on the detection data and thereby generates projection data. The console device then performs reconstruction processing based on the projection data, and creates tomographic image data or volume data based on a plurality of tomographic image data.
Helical shuttle scanning is a scanning method using an X-ray CT system. In helical shuttle scanning, helical scan (scanning by irradiating a subject with X-rays in a spiral pattern) is performed on an imaging range that includes a range corresponding to the region of interest (ROI) of the subject while a bed top on which the subject is placed is being reciprocated in the first direction and in a direction opposite to the first direction (second direction). Tomographic image data based on detection data in the first direction and the second direction acquired by the helical shuttle scanning is used for, for example, CT perfusion. CT perfusion is a technique for superimposing CT images obtained by a plurality of times of X-ray imaging of the same location. For example, an image obtained by CT perfusion is used to represent changes in contrast-enhanced cerebral blood flow by gradation to analyze the symptoms of ischemia or the like.
To superimpose a plurality of CT images as described above, in helical shuttle scanning, the trajectory of X-rays irradiated spirally to the subject is controlled to synchronize between scan in the first direction (hereinafter, sometimes referred to as “forward scan”) and scan in the second direction (hereinafter, sometimes referred to as “backward scan”). Thus, in any of the forward scan and the backward scan, the X-ray generator starts irradiation of X-rays using a position on the circular trajectory as a base point (start point). Scans of different timings, in which X-ray irradiation (and the movement of the bed top) always starts from the same position, are referred to as synchronous trajectory scan.
When CT perfusion is performed by superposing a plurality of CT images, forward scan (backward scan) has to be repeated a plurality of times under the same conditions (the position of the bed top, the number of detection data (views) to be acquired, etc.). In other words, the reproducibility of the scan is required.
If the reproducibility of the scan is poor, there may be a difference between the number of views acquired by the first forward scan and that obtained by the second forward scan. This may cause misalignment in CT images generated based on detection data obtained by the scans when the images are superimposed. As a result, it may be difficult to accurately analyze the symptoms of ischemia or the like. If misalignment has occurred in a range corresponding to the region of interest (ROI) of the subject, the effect becomes larger. In addition, since such CT images are not suitable for CT perfusion, another helical shuttle scanning is required. This leads to an increase in X-ray radiation exposure.